Conjecture on the chemical stability and corrosion resistance of Cu-Al and Au-Al intermetallics in ball bonds

Under moist conditions and in the presence of chlorine (Cl), copper ball bonds appear more prone to corrosion than gold ball bonds. One reason for this may be that Cu-Al intermetallic compounds are more susceptible to attack by moisture than Au-Al intermetallic compounds. The presence of Cl may also accelerate this effect. This article suggests that the effective heat of formation of Cu-Al and Au-Al compounds, which is a measure of stability and chemical resistance to change, may be correlated with surface energy of the various compounds. Due to lack of experimental data, surface energy of compounds is calculated in the liquid state and an assumption is made that the difference between liquid and solid compound surface energies is negligible. An initial conjecture is made that low surface energy compounds are more stable than those with high surface energy. Results of calculations for Cu-Al and Au-Al compounds suggest that CuAl₂ may be more stable than Cu₉Al₄ due to the latter having a smaller effective heat of formation (EHOF) and a higher surface energy. This is in line with experimental observations, However, the initial conjecture is modified by including work function because of the increase of surface energy of Au-Al compounds is accompanied by high stability. A further conjecture is that there is a relationship between work function, corrosion resistance and EHOF.